Abstract. We show that the frequency of summertime mid-latitude cyclones tracking
across eastern North America at 40°–50° N (the southern
climatological storm track) is a strong predictor of stagnation and ozone
pollution days in the eastern US. The NCEP/NCAR Reanalysis, going
back to 1948, shows a significant long-term decline in the number of
summertime mid-latitude cyclones in that track starting in 1980 (−0.15 a−1).
The more recent but shorter NCEP/DOE Reanalysis (1979–2006) shows
similar interannual variability in cyclone frequency but no significant
long-term trend. Analysis of NOAA daily weather maps for 1980–2006 supports
the trend detected in the NCEP/NCAR Reanalysis 1. A GISS general circulation
model (GCM) simulation including historical forcing by greenhouse gases
reproduces this decreasing cyclone trend starting in 1980. Such a long-term
decrease in mid-latitude cyclone frequency over the 1980–2006 period may
have offset by half the ozone air quality gains in the northeastern US
from reductions in anthropogenic emissions. We find that if mid-latitude
cyclone frequency had not declined, the northeastern US would have been
largely compliant with the ozone air quality standard by 2001. Mid-latitude
cyclone frequency is expected to decrease further over the coming decades in
response to greenhouse warming and this will necessitate deeper emission
reductions to achieve a given air quality goal.